The present invention relates to an image forming method using an electrophotographic procedure, and a copier, printer facsimile apparatus or similar image forming apparatus using the same.
Modern image forming apparatuses include one capable of forming a color image by using developers of different colors. This kind of apparatus is required to form toner images of three primary colors (cyan, magenta and yellow) particular to subtractive mixture on a single paper.
A color image forming apparatus may be constructed to sequentially form toner images of different colors on a photoconductive elements while sequentially transferring them to a paper one by one to complete a color image, as taught in, e.g., Japanese Patent Laid-Open Publication No. 52-73738. The problem with this type of apparatus is that the paper must have its edges clamped by a clamp mechanism and must be brought into contact with the photoconductive element several times during the course of image formation. This is not practicable without resorting to extremely complicate mechanisms. Moreover, images cannot be transferred to the clamped portions of the paper.
In light of the above, there has been proposed an image forming apparatus of the type sequentially forming toner images of different colors on a photoconductive element while sequentially transferring them to a single intermediate transfer member one above the other, and then transferring the resulting composite color image to a paper at a time. After the transfer of the color image from the intermediate transfer member to a paper, the toner remaining on the transfer member is removed by a cleaning blade or similar cleaning member. As a result, the transfer member is prepared for the next transfer of a composite color image. When the transfer member is implemented as a belt passed over a drive roller and driven rollers, it can be relatively freely arranged in a space and enhances the miniaturization of the entire apparatus.
Assume that toner of two or more different colors are combined to form a color image. Then, when toner of one color is superposed on toner of another color transferred first, the toner is sometimes scattered around the resulting color image. Much of the scattered toner exists in the vicinity of the edges of the image and causes the background to appear as if it were contaminated around the edges of the image. This kind of contamination concentrates on the portions surrounding the edges of the image. In this sense, such contamination is different from contamination which occurs evenly on the background. Particularly, when it comes to a character or similar line image, the above contamination blurs the contour of the image and thereby lowers sharpness. Moreover, when a greater amount of toner is deposited in order to increase the image density or when the edge effect available with modern digital copiers is enhanced to produce a sharp image, the contamination around the edges of an image is critical in respect of image quality.
By causing toner of two different colors to be transferred in the same amount, I found that the contamination around an image, as distinguished from background contamination, is attributable to the toner overlying the toner transferred first, i.e., the overlying toner partly drops from the underlying toner. I conducted a series of researches and experiments in order to determine the mechanism which brings about the above occurrence, as follows.
The factors effecting the mechanism is the history of the intermediate transfer belt, among others, and the history of the toner. As to the history of the belt, the belt is usually movable in contact with the surface of the photoconductive belt in order to receive toner images from the element. The belt is a semiconductor consisting of, e.g., a fluorine-contained resin and carbon or similar conductive substance mixed therewith; carbon sets up an electrical path for conducting a bias for image transfer. When the bias is applied to the belt, the belt electrostatically attracts the toner away from the photoconductive element and causes it to deposit thereon. As the belt moves away from position where it contacts the photoconductive element, a charge is induced due to separation discharge. This charge remains on the belt up to the time of the next image transfer. In this connection, charges remaining on the photoconductive element are dissipated by cleaning every time an image of one color is formed on the element. As a result, an electric field is generated on the belt at the boundary where a portion with the charge and a portion without the charge adjoin each other. This electric field causes the toner to be transferred next to electrostatically deposit on the belt easily.
As for the history of the toner, the toner transferred to the belt first is influenced by the transfer bias necessary for the electrostatic adhesion thereof to the belt. A bias potential assigned to the toner to be transferred next is higher than the transfer bias assigned to the toner transferred first. As a result, the two different toner are apt to repulse each other. This is partly because they are of the same polarity.
Experiments showed that the scattering of toner attributable to the above factors depends on the image forming mode also. Generally, a copy image forming mode and a printer image forming mode are available with an image forming apparatus. In the copy image forming mode, the apparatus serves as a copier and reads a document image, generates separated color image data representative of the image, and writes the image data. In the printer image forming mode, the apparatus serves as a printer and directly uses a signal received from a computer as data to write. In the copy image forming mode, the image data generated by color separation is 70% to 80% of the actual colors of a document image because usually the data is read out of the document image by an analog system. By contrast, in the printer image forming mode, 100% of image data is available because image data is output by a digital system. Hence, the amount of scattering of toner to occur when toner of different colors are superposed and, therefore, the reproducibility of an image depends on the kind of image data.